Control of spin injection by direct current in lateral spin valves

The spin injection and accumulation in metallic lateral spin valves with transparent interfaces are studied using dc injection current. Unlike ac-based techniques, this allows the investigation of the effects of the direction and magnitude of the injected current. We find that the spin accumulation is reversed by changing the direction of the injected current, whereas its magnitude does not change. The injection mechanism for both current directions is thus perfectly symmetric, leading to the same spin injection efficiency for both spin types. This result is accounted for by a spin-dependent diffusion model. Joule heating increases considerably the local temperature in the spin valves when high-current densities are injected ($\sim 80–105\text{K}$ for $1–2\times {10}^7\text{A}{\text{cm}}^{-2}$), strongly affecting the spin accumulation.

Figure 1

(Color online) (a) Scanning electron microscope image of a lateral spin valve with a schematic illustration of spin injection, accumulation, and detection in a nonlocal measurement. Thick vertical arrows indicate the state of the magnetization in the ferromagnetic electrodes. Thin vertical arrows represent the accumulation of injected spins in the nonmagnetic strip. (b) Normalized NLSV signal measured in a Py/Cu/Py lateral spin valve at 4.2 K and 0.5 mA with a dc reversal technique, while sweeping the magnetic field in the direction given by the thin arrows. Thick vertical arrows indicate again the magnetic alignment of the electrodes.

Figure 2

(Color online) Normalized NLSV signals measured in a (a) Py/Cu/Py and a (b) Co/Al/Co lateral spin valve at 4.2 K and 0.5 mA while sweeping the magnetic field in the direction given by the horizontal arrows. Signals measured at positive and negative currents are plotted as a solid and a dotted line, respectively.

Figure 3

(Color online) Normalized NLSV signals measured in a (a) Py/Cu/Py and (b) Co/Al/Co lateral spin valves at 4.2 K as a function of dc for a parallel (red solid lines) and antiparallel (blue dotted lines) magnetic alignments of the electrodes. The same NLSV signal averaged for positive and negative dc currents, $\frac{V\left(+I\right)-V\left(-I\right)}{2\left|I\right|}$, is plotted as a solid (dotted) black line for a parallel (antiparallel) configuration as a function of the absolute value of the current. (c) and (d) show NLSV signal difference, $\left(V_{\text{P}}-V_{\text{AP}}\right)/\left|I\right|=\Delta V/\left|I\right|$, as a function of dc, calculated from data in (a) and (b), respectively. Note the opposite sign in the vertical scale for positive and negative dc.

Figure 4

(Color online) The spatial dependence of the electrochemical potential $\mu$ for spin-up and spin-down electrons in the nonlocal configuration of a lateral spin valve [FM injector (y1, left), normal metal (x, middle) and FM detector (y2, right)] when injecting (a) negative and (b) positive currents. Solid (dashed) lines in the FM detector correspond to $\mu$ when the magnetic alignment is parallel (antiparallel). The corresponding schematic illustration of spin injection, accumulation, and detection for negative and positive dc is drawn to the right.

Figure 5

(Color online) NLSV signal difference, $\left(V_{\text{P}}-V_{\text{AP}}\right)/\left|I\right|=\Delta V/\left|I\right|$, measured in [(a) and (c)] Py/Cu/Py and [(b) and (d)] Co/Al/Co lateral spin valves with a dc reversal technique [(a) and (b)] as a function of the current magnitude at 4.2 K and [(c) and (d)] as a function of temperature at 0.2 mA. Horizontal arrows and dotted vertical lines are used to estimate the local temperature of the lateral spin valves when 5 mA are applied. Note that the devices shown here are different from the ones shown in the previous figures.